A Black Swan in this context would be a shock to the system that no one saw coming nor could see coming. For instance a change in the sun which would drastically warm or cool the earth far beyond the effects of CO2. Maybe a natural disaster or an industrial accident that had a cascade effect that would reach beyond what anyone could have seen coming. You can't do anything about either of these things but that won't stop people from suggesting it could have been avoided with the right (or more of): "fill in the blank with your favorite solution to all problems in life."